Make sure all your PC's drivers are installed, most importantly the chipset drivers, reboot, and then the rest of the drivers.
R. Andrew Koffron
look up the manufacturer's hot keys for video display it's probably something like fn+F5 that rotates the system through various monitor configurations.
look for the little pictures on the F1-F12 keys of monitors
like 90% of the time when clients ask me about this, I do like 10 fn+FKeys and tada.. it's working.